Process of producing corrosion-resisting coating on iron and steel and products thereof



Patented Jan. 11, 1927.

UNITED STATES PATENT OFFICE.

CORPORATION, OF INDIANAPOLIS, INDIANA, A CORPORATION OF INDIANA.

rnoonss or rnonuome oonnosron-nnsrsrme COATING on men AND s'rnnil AND rnonuors THEREOF.

No Drawing. Original application filed June 5, 1924, Serial No. 718,080. Divided and. this application filed November 22, 1926, Serial No. 150,138.

I have found that by applying a very thin coating of chromium over the coatings of other metals upon iron and steel, products having very desirable corrosion-resisting properties may be produced, and further that the chromium coating can be produced in such form that further mechanical finishing thereof is unnecessary.

In Patent No. 1,545,196 to Pierce and Humphries there is described a method of producing extremely thin continuous and c0- herent coatings of chromium upon other metals. Such chromium coatings have excellent corrosion-preventing properties. U nder certain circumstances, however, it is unwise to trust solely to the chromium coating. The present invention therefore relates to the application of multiple coatings of certain metals, the outermost bf which coatings is chromium, and to the products so produced.

The invention embraces quite a variety of combinations of coatings among which may be mentioned by way of example the following:

Chromium on copper on cadmium on 1ron or steel;

Chromium on copper on zinc on 1ron or steel;

Chromium on brass on cadmium on iron or steel;

Chromium on brass on zinc on iron or steel;

The coatings may be applied by any suitable electroplating methods but with regard to'the chromium coating it is desirable from a con'imercial standpoint that a process be used which will give ,a lustrous coating at low current density, otherwise, for instance if a high current density is required or if the coating is not lustrous, some of the value of the composite coating from a commercial standpoint is lost. I therefore prefer to use the process of chromium plating which 4:) forms the subject matter of the Pierce and Humphries patent above referred to. Said process consists generally in electro-depositing chromium at a relatively low current density, say from 14 up to 125 amperes per square foot, but preferably in the neighborhood of 14 to40 amperes per square foot, from a bath containing essentially chromic acid, ammonium sulfate and hydrated chromic hydroxid dissolved in water at room or slightly elevated temperature, using either inert or chromium anodes.

The bath may be prepared by mixing a solution of chromic acid in water with a solution of the reaction products of chromic sulfate and ammonium hydroxide in water. The quantity of chromic acid used may vary from 80 to. 450 grams per liter of the final solution. The chromic sulfate is used in quantity amounting to 1 to 2 per cent of the chromic acid and the ammonium hydroxide preferably in quantity about chemically equivalent to the chromium sulfate. The invention is not limited to the use of the particular bath and process for applying the chromium coating described above. The so-called Sargents solution for which somewhat higher current densities are recom- I mended may be used.

The following examples illustrate the process:

Iron or steel articles, properly prepared mechanically for finish and by ordinary plating-room cleaning methods to receive an electrodeposited coating, are plated ina zinc cyanide bath with a coating of zinc approximately, .0001 inch or more in thickness,

rinsed in Water and given an electrodeposited coating ot copper or brass from a copper cyanide or standard brass plating bath, subjected to mechanical buffing or to a bright acid dip to give the copper or brass coating a high luster, and then coated with chromium by means of the Pierce and Humphries chromium plating process described above.

The multiple coatings with chromium on the outer surface produced in accordance with this invention give remarkable atmospheric corrosion and acid fume resisting properties to the coated articles. A coating of chromium on copper on zinc on steel with the zinc coating .0001 inches thick and the copper coating .0001 inches or less thick and the chromium coating .00006 inches or less thick y computation, to the standard salt-spray test for 80 hours showed no evidence 0 .ofa

' by coatings of metals such I or breaks in either the iron or nust streaks or areas,

or pm holes, and the original bright luster of the chromium coating was undimmed.

For comparison it is noted that standard nickel on steel and nickel on copper on steel platings subjected to the same salt spray test failed in 10 hours, while a nickel on ca ium on steel coating failed in 24 hours. The chromium coated articles not only withstand corrosion longer than the other standard coatings referred to, but have the additional and very important further advantage that they retain their luster and do not become dulled or tarnished.

'echromium coatings should not be of less than a computed thickness of .00006 inches, because with coatings of less than this thickness there is a tendency for the under coatings to show through after standing, there being evidence of chromium absorption.

Cf the composite coatings those in which the chromium over-coating is deposited directly upon an under-coatin of copper or brass are preferred because c romium coatings upon copper and brass show a reater tendency to be lustrous and the pro uction lustrous finish directly obviates the more or less expensive mechanical coloring treatment which otherwise would be necessary in order to produce lustrous finishes.

e corrosion-resisting properties of the composite coatings of the present invention may be explained in part by the. fact that the chromium coatings are very hard and protect the undercoating against abrasion. A further'explanation is as follows.

on iron is protected againstcorrosion as cadmium and zinc which are electropositive to iron the protection is given by the preferential J oxidation of the coating metal, while when coatings of nickel alone or nickel on copper on iron are used, the protective action is due solely to the continuity of the coatings since both copper and nickel are electronegative to iron. In the case of coatings of nickel on cadmium or zinc on iron the intermediate metal, cadmium or zine which is electropositive to both the ironand nickel, is very rapidly used up due to electrolytic action in case pin holes nickel occur. Such conditions do not exist inthe coatings of the present invention. For instance in the case of the coating of chromium on copper on cadmium on iron the relationship is as follows. Cadmium is positive to iron,

copper is'negative to the cadmium and the Y c romium is electropositive to per with which e iron. Throughout the specification and in the both the copit is in contact and also to after being subjectedor chromium. By el top of the table.

claims, I mean by an electropositive metal,

a metalfielectroposltive to iron, and above iron inthe electromotive series, such as zinc, ectronegati-ve metals, I mean metals below iron in the electromotive series, such metals being nickel, copper, etc.

There is a discrepancy in the literature with regard to the designation of elements in theelectromotive series and for purposes of clarity and consistency, the following table, taken from Mellors Modern Organic Chemistry (1917 edition, page 363) is appended.

Sodium Cobalt Magnesium Nickel Chromium Tin Manganese Lead inc Hydrogen Cadmium Copper. Iron The more electropositive metals are at the Thus zinc is electropositive to iron whereas tronegative to iron.

Not all of the elements listed in the Mellor table have been listed herein. The purpose of the table is to and negativity of the metals towards iron.

hus,.chromium on copper on cadmium on iron or steel bears the relation, potentially speaking, of a metal electropositive to iron superposed on an electronegative metal which is in turn superposed on an electropositive metal plated on an iron or steel base.

This application is a division of m copending application Serial N 0. 718,080, filed June 5, 1924.

v I claim:

1. Process ,of protecting iron and steel articles against corrosion which comprises applying to the surfaces thereof successive coatings of a metal which is electropositive to iron, a metal which is electronegative to said first named coating metal, and finally a coating of chromium.

2. Process of protecting iron and steel articles against corrosion which comprises applying to the surfaces thereof successive coatings of a metal which is eleetropositive to iron, a metal which is electronegative to said first named coating metal and to chromium, and finally a coating of chromium.

3. Process of protecting iron and steel articles against corrosion which comprises applying to the surfaces thereof successive coatings of cadmium, copper and chromium.

4. As an article of manufacture a piece of iron or steel having a composite coating comprising a layer of a metal electropositive to iron, a superposed layer of a metal electronegative to said electropositive layer and copper or nickel is elecshow the relative positivity clcct-ronegative to chromium and an outer layer of chromium.

5. As an article of manufacture a piece of iron or steel having a composite coating 5 comprising a layer of cadmium a layer of copper on said layer of cadmlum, and a.

layer of chromium on said layer of copper.

6. As an article of manufacture, a pieceof ron, a superposed layer of a metal electronegative to said electropositive layer and an outer layer of chromium.

Nov. 29th, 1926.

CHAD H. HUMPHRIE'S'. 

